The present invention relates generally to sensing devices. More specifically, the present invention relates to a fiber-optical sensor for detecting electric arc-discharges. The sensor is used in a system having a source of testing light and photoreceiver.
Optical waveguides can detect and transmit light pulses occurring within the acceptance angle of the waveguides. The acceptance angles of known optical waveguides usually lie between 18.degree. and 46.degree., and are therefore almost too small for many possible applications, for example as sensors for detecting electric arc-discharges in enclosed high-voltage switching systems. However, optical waveguides have the advantages of high electromagnetic compatibility and good potential-isolation, so that there is a desire to use them in many cases.
For safety and reliability reasons, it is necessary to check the functional serviceability of the sensors on a continuous basis. These checks are conventionally carried out with the aid of testing-light pulses, which are led, by a second optical waveguide, into the space which is to be monitored. The testing light pulses are then radiated onto the detection point of the optical waveguide, within the acceptance angle.
On account of the distance and of the radiating angle, the optical waveguide serving as the sensor receives only a small portion of the testing light again. The greater the distance between the two optical waveguides, the smaller is the quantity of testing light which is radiated into the sensor. Since a minimum radiant flux is necessary for reliable detection, and the luminous intensity decreases as the fourth power of the distance, the two optical waveguides must be adjusted so that they are a short distance apart and, as nearly as possible, on one line.
The radiation source emitting the light pulses which are to be detected must also be located within the acceptance angle of the optical waveguide serving as the sensor, that is to say, it must generally be located between the two optical waveguides. As a result of this requirement, considerable problems arise, related purely to design, since either the radiation source is so large that it cuts off the testing light, or there is otherwise the danger of the arc-discharge itself damaging the optical waveguides, for example in the case of a high-voltage switch.
One object underlying the present invention is to design a fiber-optical sensor for detecting electric arc-discharges in such a manner that light pulses can be received from a very large acceptance angle, without problems arising in connection with the testing light.
This object and others are achieved by an optical waveguide, which is heavily curved in its detecting or active zone. At the end of the optical waveguide, the testing light and discharge light are respectively converted into electrical signals, and processed, an optical/electrical unit being used for this purpose. Differentiation between testing light and radiated light is most simply effected on the basis of the intensity of the radiation, the duration of the light pulses, or the moment in time at which the light pulses are received.
Differentiation between testing light and light from the arc-discharge can also be effected by a continuously-operating source of testing light, located at the beginning of the optical waveguide. The differentiation can also be effected by an optical filter, located at the end of the optical waveguide and matched to the testing light, by a first photoreceiver for the testing light, and by a second photoreceiver for the light from the arc-discharge.
The invention utilizes the knowledge that light can emerge from the optical waveguide, or enter the optical waveguide, at the location where the optical waveguide is heavily curved. The proportion of the light which emerges, just as the proportion of the light which is coupled-in, is a function of the radius of curvature, and of the length over which the optical waveguide is curved. The radius and the length must accordingly be adjusted so that, of the testing light which is fed in at one end of the optical waveguide, sufficient energy still arrives at the exit to permit reliable detection, and that sufficient light is absorbed, from the electric arc-discharge, that it is likewise reliably detected. To detect the light from the arc, it is sufficient to design the active zone of the optical waveguide as a sharp kink, or as a knot.
In a further embodiment of the present invention, the invention utilizes the fact that an electric arc-discharge not only generates light, but also generates sound waves. In accordance with their travel time, these sound waves reach the sensor location later than the light waves, and modulate the testing light which is guided within the optical waveguide. When operating with coherent light, this modulation results from a change in the phases of the modes, and, when operating with non-coherent light, it takes place as a result of a change in the quantity of light being radiated from the curved surfaces of the optical waveguide. However, in both cases, the sound waves result in the intensity of the testing light being modulated at the end of the optical waveguide. Suitable optical filters are used to separate the modulated testing light from the light which is coupled-in from the arc-discharge.
Depending on the amount of energy required, a laser or a luminescent diode can be used as the sources of testing light, or even a photoflash lamp can be used. If a photoflash lamp is used as the source of testing light, it is necessary for the source of testing light to be time-synchronized with the testing-light receiver.
The active zone of the optical waveguide is advantageously designed in the form of a spiral, thus enabling the light from the arc-discharge to be detected from virtually all sides.
The sensors according to the invention are preferentially used in high-voltage switching systems, or in transformer tanks.
Further developments of the invention, and the advantages thereof, are evident from the description, in the text which follows.